Oxy/fuel combustion system with little or no excess oxygen

ABSTRACT

The disclosure includes a combustion system including a primary reactor arranged and disposed to receive a solid fuel and a first oxygen stream and deliver a first substantially gaseous product and a substantially solid or molten product, a secondary reactor in fluid communication with the primary reactor, and a furnace in fluid communication with the secondary reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to Application No. ______, entitled“COMBUSTION SYSTEM WITH STEAM OR WATER INJECTION”, Attorney Docket No.07238 USA, filed contemporaneously with this Application on Sep. 26,2008, assigned to the assignee of the present disclosure and which isherein incorporated by reference in its entirety, Application No.______, entitled “COMBUSTION SYSTEM WITH PRECOMBUSTOR”, Attorney DocketNo. 07255 USA, filed contemporaneously with this Application on Sep. 26,2008, assigned to the assignee of the present disclosure and which isherein incorporated by reference in its entirety, Application No.______, entitled “OXY/FUEL COMBUSTION SYSTEM WITH MINIMIZED FLUE GASRECIRCULATION”, Attorney Docket No. 07257 USA, filed contemporaneouslywith this Application on Sep. 26, 2008, assigned to the assignee of thepresent disclosure and which is herein incorporated by reference in itsentirety, Application No. ______, entitled “CONVECTIVE SECTIONCOMBUSTION”, Attorney Docket No. 07254 USA, filed contemporaneously withthis Application on Sep. 26, 2008, assigned to the assignee of thepresent disclosure and which is herein incorporated by reference in itsentirety, Application No. ______, entitled “OXY/FUEL COMBUSTION SYSTEMHAVING COMBINED CONVECTIVE SECTION AND RADIANT SECTION”, Attorney DocketNo. 07247 USA, filed contemporaneously with this Application on Sep. 26,2008, assigned to the assignee of the present disclosure and which isherein incorporated by reference in its entirety, Application No.______, entitled “PROCESS TEMPERATURE CONTROL IN OXY/FUEL COMBUSTIONSYSTEM”, Attorney Docket No. 07239 USA, filed contemporaneously withthis Application on Sep. 26, 2008, assigned to the assignee of thepresent disclosure and which is herein incorporated by reference in itsentirety, Application No. ______, entitled “COMBUSTION SYSTEM WITHPRECOMBUSTOR”, Attorney Docket No. 07262Z USA, filed contemporaneouslywith this Application on Sep. 26, 2008, assigned to the assignee of thepresent disclosure and which is herein incorporated by reference in itsentirety, and application Ser. No. 12/138,755, entitled “OXYGEN CONTROLSYSTEM FOR OXYGEN ENHANCED COMBUSTION OF SOLID FUELS”, Attorney DocketNo. 07162 USA, filed Jun. 13, 2008, assigned to the assignee of thepresent disclosure and which is incorporated herein by reference in itsentirety.

FIELD OF THE DISCLOSURE

The present disclosure is directed to an oxy/fuel combustion system andmethod. In particular, the present disclosure is directed to anoxygen-enriched solid fuel combustion system and method.

BACKGROUND OF THE DISCLOSURE

As the world-wide demand for electric power continues to grow, so doesthe urgency for developing sustainable and environmentally responsiblemethods for power generation. Considering the abundance of global coalreserves, the recent emergence of oxygen fired coal technology, which isideally suited for CO₂ capture, will be called upon to play a leadingrole. There is consequently a need to develop refinements to thetechnology which will improve its energy efficiency and reduce its costof implementation. The disclosure disclosed herein is directed towardthe accomplishment of this objective.

Due to slower overall combustion kinetics, excess oxygen requirementsfor coal combustion are generally much higher than for gaseous andliquid fuels. For example, whereas the stoichiometric ratio (i.e. ratioof actual to theoretical minimum O₂ required) for gaseous phasecombustion (e.g. natural gas) is often 1.05 (5% excess) or less, thestoichiometric ratio for coal combustion is more typically in thevicinity of 1.2 (20% excess).

Therefore, there is an unmet need to provide efficient methods andsystems for generating energy by solid fuel combustion in oxygen-basedsystems.

SUMMARY OF THE DISCLOSURE

This disclosure provides a device and method for burning solid fuel,such as coal, with oxygen and recycled flue gas in a multi-stagecombustion process.

According to an embodiment, a combustion system includes a primaryreactor arranged and disposed to receive a solid fuel and a first oxygenstream and deliver a first substantially gaseous product and asubstantially solid or molten product, a secondary reactor in fluidcommunication with the primary reactor, and a furnace in fluidcommunication with the secondary reactor. In the embodiment, thesecondary reactor is disposed to receive a second oxygen stream therebyconverting the first substantially gaseous product from being oxygendeficient upon entering the secondary reactor to oxygen rich uponexiting the secondary reactor.

According to another embodiment, a method of operating a combustionsystem includes providing a primary reactor arranged and disposed toreceive a solid fuel and a first oxygen stream and deliver a firstsubstantially gaseous product and a substantially solid or moltenproduct, providing a secondary reactor in fluid communication with theprimary reactor, providing a furnace in fluid communication with thesecondary reactor, and determining a stoichiometric ratio selected fromthe group consisting of the stoichiometric ratio of the primary reactor,the stoichiometric ratio of the secondary reactor, the stoichiometricratio of the furnace, and combinations thereof. In the embodiment, thesecondary reactor is disposed to receive a second oxygen streamconverting the first substantially gaseous product from being oxygendeficient upon entering the secondary reactor to oxygen rich uponexiting the secondary reactor.

An advantage of the present disclosure is the ability to achievesubstantially complete combustion of coal with a reduced amount of O₂.

Another advantage of the present disclosure is the ability to produce aproduct gas with high CO₂ purity.

Yet another advantage of the present disclosure is the ability to removefly ash and other contaminants resulting in reduced fouling.

Further aspects of the method and system are disclosed herein. Thefeatures as discussed above, as well as other features and advantages ofthe present disclosure will be appreciated and understood by thoseskilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a graphic representation of the effect of thestoichiometric ratio on flue gas CO₂ in oxygen-enriched coal combustion.

FIG. 2 illustrates a diagrammatic representation of an exemplaryembodiment of a combustion system according to the disclosure.

FIG. 3 illustrates a diagrammatic representation of a portion of acombustion system according to an embodiment of the disclosure.

FIG. 4 illustrates a diagrammatic representation of a portion of acombustion system according to another embodiment of the disclosure.

FIG. 5 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 6 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 7 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 8 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 9 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 10 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 11 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 12 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 13 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 14 illustrates a diagrammatic representation of a portion of acombustion system according to still another embodiment of thedisclosure.

FIG. 15 illustrates a diagrammatic representation of an alternateexemplary embodiment of a combustion system according to the disclosure.

FIG. 16 illustrates a diagrammatic representation of an alternateexemplary embodiment of a combustion system according to the disclosure.

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure now will be described more fully hereinafter withreference to the accompanying drawings, in which a preferred embodimentof the disclosure is shown. This disclosure may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the disclosure to those skilled in the art.

Certain embodiments of the present disclosure include systems andmethods for combusting solid fuel. As used herein, the term “solid fuel”and grammatical variations thereof refers to any solid fuel suitable forcombustion purposes. For example, the disclosure may be used with manytypes of carbon-containing solid fuels, including but not limited to:anthracite, bituminous, sub-bituminous, and lignite coals; tar; bitumen;petroleum coke; paper mill sludge solids and sewage sludge solids; wood;peat; grass; and combinations and mixtures of all of those fuels. Asused herein, the term “oxygen” and grammatical variations thereof refersto an oxidizer having an O₂ concentration greater than that ofatmospheric or ambient conditions. As used herein, the term “oxy/coalcombustion” and grammatical variations thereof refers to coal combustionin oxygen, the term “air/coal combustion” and grammatical variationsthereof refers to coal combustion in air, the term “oxy/fuel combustion”and grammatical variations thereof refers to fuel combustion in oxygen,and the term “air/fuel combustion” and grammatical variations thereofrefers to fuel combustion in air. As used herein, the term “combustionfluid” and grammatical variations thereof refers to a fluid formed fromand/or mixed with the products of combustion, which may be utilized forconvective heat transfer. The term is not limited to the products ofcombustion and may include fluids mixed with or otherwise travelingthrough at least a portion of combustion system. Although not solimited, one such example is flue gas. As used herein, the term“recycled flue gas” and grammatical variations thereof refers tocombustion fluid exiting the system that is recirculated to any portionof the system. As used herein, the term “flue gas recycle” andgrammatical variations thereof refers to a configuration permitting thecombustion fluid to be recirculated.

FIG. 1 illustrates a graphic representation of the effect of thestoichiometric ratio on flue gas CO₂ during the combustion of coal withoxygen; hereinafter referred to as oxygen fired coal or oxygen firedfuel combustion. In the context of air fired fuel combustion, operationwith a relatively high stoichiometric ratio results in relatively highstack sensible enthalpy losses and fan power requirements, the latterbeing typically only a fraction of a percent of gross power generationof the steam turbine. However, the penalty of relatively highstoichiometric ratio operation during oxygen fired fuel combustion ismuch greater. This is principally due to higher power requirements forcompression within the Air Separation Unit (ASU), as well as the needfor higher capacity ASU equipment, leading to higher capital costs. As abasis for comparison, the ASU compressor power is typically severalpercent of gross generation, rather than the fraction of a percent forfan power in air fired fuel systems.

Another reason for the greater need for reducing the stoichiometricratio during oxygen fired coal boiler operation is that the products ofoxygen fired coal combustion comprise principally CO₂, H₂O, and severalinert species; the most abundant among them being O₂. Hence, as thestoichiometric ratio is reduced, the CO₂ concentration of the productgas stream increases, reducing the burden of downstream equipmentrequired for CO₂ purification. Moreover, the total volume of gasesprocessed in the CO₂ purification is lowered, leading to lower capitaland operating costs. It should be noted that CO₂ compression powerrequirements may be of the same order of magnitude as for compressionwithin the ASU.

The challenge in low stoichiometric ratio operation during coalcombustion is in attaining high combustion efficiency. Emissions of COand unburned carbon are known to increase substantially as thestoichiometric ratio is lowered beneath about 1.2, leading to poorthermal efficiency, a higher propensity for fouling, potentiallyhazardous conditions within the plant and a higher collection burden ondownstream particulate control equipment. Known systems do not providemeans for generating electric power in oxygen fired coal boilers withsimultaneously low stoichiometric ratio and high thermal efficiency.

Specifically, FIG. 1 illustrates the variation of CO₂ purity and balanceof inert gases, principally O₂, SO₂ and N₂ formed from fuel nitrogen,with stoichiometric ratio for oxygen fired fuel combustion of a typicallow sulfur (˜1 wt %) coal. As illustrated in FIG. 1, lowering thestoichiometric ratio from about 1.2 to about 1.05 reduces theconcentration of inerts from about 18% to about 6% and increases the CO₂concentration from about 81.5% to about 94%. Note that allconcentrations are presented on a dry basis. Methods of combustionaccording to the present disclosure provide low excess oxygen and serveas a method for reducing the size of, or potentially eliminating, CO₂purification equipment. Moreover, both the size/extent of purificationequipment and the respective operating costs may be lower due to thesmaller volume of gases to be removed. The reduction or removal of thisequipment may lead to significant savings, particularly in relation toflue gas compression required for efficient CO₂ transport, for exampleto an external pipeline, where pressures of 1000 psia or greater,depending upon end use, may be required.

The combustion system according to certain embodiments operates atstoichiometric ratios of about 1.05 or less. Known coal combustionsystems typically operate at a stoichiometric ratio of about 1.2 orgreater. Operation of a solid fuel combustion system of a stoichiometricratio of less than about 1.05 results in additional features beingdesired for efficient combustion system operation. For example, it isdesirable to provide additional residence time between the solid fueland oxidizer to facilitate complete evolution of fuel carbon into agaseous phase. It is also desirable to provide oxygen instead of air asan oxidizer in order to attain sufficiently high temperature within theprimary reactor to melt ash constituent of the solid fuel, and toincrease the combustion reaction rates throughout the system. It isfurther desirable to provide a controlled environment for mixing ofoxygen and fuel evolved from the solid to the gaseous phase in order tominimize the required residence time for complete burnout and avoid hightemperature damage that could otherwise result during oxygen fired fuelcombustion. It is also desirable to provide close-coupled combustioninstrumentation to provide feedback with which to control the combustionprocess.

FIG. 2 illustrates a diagrammatic representation of an exemplaryembodiment of a combustion system according to the disclosure.Specifically, FIG. 2 illustrates an embodiment of an oxygen fired coalcombustion system 202 required to facilitate efficient, low-excess O₂operation. As illustrated, oxygen fired coal combustion system 202includes a primary reactor 204, a secondary reactor 206, and a furnace208 (which includes, but is not limited to a combustion chamber).Primary reactor 204 is in fluid communication with secondary reactor206. Secondary reactor 206 is in fluid communication with primaryreactor 204 and furnace 208. Furnace 208 is in fluid communication withsecondary reactor 206.

In one embodiment, primary reactor 204 may be a slaggingcombustor/gasifier such as, for example, a slagging cyclone. The type ofreactor is selected to provide the ability to achieve relatively longsolid particle residence times and withstand high gas temperatures, thuspromoting efficient gasification and/or combustion of the feed coal withlittle or no carbon residue. Residual solid material 212, which includesash, may be removed as a viscous slag and delivered into the boilerwhere it is captured in the bottom of furnace 208, thus minimizing theconcentration of particulate in flue gas. Residual solid material 203may be integral, as illustrated in the embodiment of FIG. 2.Alternatively, slag collection may be separate from furnace 208 asillustrated in the embodiment of FIG. 16. In another embodiment, primaryreactor 204 may be a slagging cyclone which is operated with less thanthe stoichiometric amount of oxygen required for complete combustion ofthe fuel. That is, the stoichiometric ratio of the fuel and oxidantintroduced into the primary reactor is less than 1.0. More preferably itis less than 0.95, and still more preferably it is the range of 0.3 to0.95, wherein the lower limit is sufficiently high to ensure that theslag can be maintained in a molten state and the higher limit isdictated by the preference, for control purposes, to maintain at least aminimal amount of oxygen that must be added outside the primary reactorto complete the low excess oxygen combustion process. Suitablearrangements of slagging combustor/gasifiers include, for example, thearrangement disclosed in U.S. Pat. No. 5,291,841, and U.S. Pat. No.5,052,312, which are both incorporated herein by reference in theirentirety, while not intending to be limiting.

Referring to FIG. 2, primary reactor 204 is arranged and disposed toreceive fuel 210 and oxygen 205. Fuel 210 may be crushed or pulverizedfuel in a proportion dictated by the need to attain a specifictemperature for primary reactor 204. The fuel may also be conveyed by asmall amount of a transport fluid. Although not so limited, thetransport fluid may be air, CO₂, N₂, liquid or gaseous H₂O, recycledflue gas, or combinations thereof. The desired temperature of primaryreactor 204 may be determined based upon the slag melting temperature,and monitored using commercially available instrumentation fortemperature measurement. These temperatures permit conversion ofessentially all of a solid carbon fuel into a gaseous phase. Primaryreactor 204 is arranged and disposed to permit a residual solid material212 to be expelled from it (i.e. slag). Generally, residual solidmaterial 212 is in a molten state and is essentially free of residualcarbon. Primary reactor 204 is arranged and disposed to permit apartially combusted gaseous product 216 to be expelled from it intosecondary reactor 206. In one embodiment, residual solid material 212 isexpelled separate from partially combusted gaseous product 216 expelledfrom primary reactor 204.

In another embodiment, oxygen fired coal combustion system 202 includesa recirculator 218 arranged and disposed to permit a recycled flue gas214 to be transported from a recirculator 218 to primary reactor 204. Inthe embodiment, recycled flue gas 214 is injected into primary reactor204 with a stream of primary oxygen 207 and crushed or pulverized fuelin a proportion dictated by the preference to maintain a predeterminedtemperature in primary reactor 204 in excess of residential solidmaterial 212 temperature and convert essentially all of the solid carboninto a gaseous phase. Other process constraints such as moderation ofboiler radiant heat flux and final steam temperatures may alsocontribute to the selection of primary reactor 204 operating conditions.As discussed below, embodiments of the present disclosure also includestreams of tertiary oxygen 225 and quaternary oxygen 227. In addition,embodiments include additional streams of recycled flue gas 215, 219.

In one embodiment, secondary reactor 206 may be arranged and disposed toreceive recycled flue gas 214 from recirculator 218 thereby adding it topartially combusted gaseous product 216.

The placement of the streams of the oxygen illustrated in FIGS. 3through 8 may permit increased control of oxygen fired coal combustionsystem 202. Referring to FIG. 3, secondary reactor 206 includes an innerstream comprised of partially combusted gaseous product 216 expelledfrom primary reactor 204. Partially combusted gaseous product 216 mayalso include trace amounts of fully or partially combusted particulates.In one embodiment, secondary reactor 206 may further include at leastone stream of secondary oxygen 224 (see also FIG. 2) bounding partiallycombusted gaseous product 216 as it enters secondary reactor 206.Reactants introduced into secondary reactor 206 in this manner affordseveral advantages. Primarily, the configuration of secondary reactor206 allows for control of the extent of reaction and the momentum of thereacting gases. This control or operation is exerted principally throughthe relative amounts of fuel and oxygen present within secondary reactor206, the manner in which reactants are introduced into secondary reactor206, and the size of secondary reactor 206.

For example, as the amount of oxygen in secondary oxygen 224 isincreased relative to fuel 210, equilibrium favors an increase in theextent of fuel oxidation. The increase in fuel oxidation should lead togreater energy release prior to the gases discharging into furnace 208where the gases are diluted with furnace 208 gases. This in turntranslates to higher mixture temperature and faster chemical kineticswithin secondary reactor 206. The faster reaction speeds will furtherincrease the extent of reaction (i.e. shortening the approach toequilibrium). Moreover, the higher temperature gas will possess a lowerdensity and, hence, a higher velocity. Therefore it will also possessgreater momentum as it enters furnace 208.

As another illustration, if the length of secondary reactor 206 isincreased, the time available for reaction also increases. Consequently,the energy release, mixture temperature, velocity and momentum shouldagain increase.

As a further example, the manner of mixing illustrated in FIG. 3 has twoprincipal advantages. First, arranging the stream of secondary oxygen224 around partially combusted gaseous product 216 entering from primaryreactor 204 creates a layer of relatively cool gas adjacent to secondaryreactor 206 wall to cool and protect the wall from high temperaturedamage. The potential for high temperature damage is much greater foroxygen fired fuel combustion relative to air fired fuel combustion sincethe flame temperature attained using oxygen can be as much as 1500° F.higher than the temperature attained with air, and the reaction ratescan be increased by a factor of 10 or more relative to combustion withair. Further, by streamlining partially combusted gaseous product 216 atsecondary reactor 206 intake such that the flow vectors of oxygen andprimary reactor 204 effluent are directed essentially along the axis ofsecondary reactor 206, transverse mixing can largely be eliminating.That is, the initial mixing between oxidizer (e.g. oxidizer) andreactant occurs can be confined to a shear layer between the two fluids.By largely eliminating transverse mixing, it is possible to minimize thedeposition of particulate onto the reactor walls (i.e. carry-over fromprimary reactor 204) and additionally minimize the contact of hightemperature gases, which thereby further reduces the risk of hightemperature damage.

Referring to FIG. 4, in another embodiment, secondary reactor 206 mayinclude recycled flue gas 214 provided or injected to bound or otherwisesurround the stream of secondary oxygen 224. The advantage of this isthat it provides an additional buffer to protect against hightemperature gases and/or particulate coming into contact with the wallsof secondary reactor 206 while permitting intimate contact between theoxygen and products from primary reactor 204. In addition, it permitsmore aggressive reactant mixing within secondary reactor 206 withoutincreasing the risk of high temperature damage.

As illustrated in FIG. 5, the use of recycled flue gas 214 in secondaryreactor 206 may be mixed with stream of secondary oxygen 224 by means ofa swirler generator 502. In other embodiments, other techniques known inthe art for enhancing mixing stream of secondary oxygen 224 andpartially combusted gaseous product 216 may be included. Mixing thestream of secondary oxygen 224 and partially combusted gaseous product216 provides control for adjusting the momentum of the gases dischargedfrom secondary reactor 206 into the furnace 208. That is, adding more ofthe recycled flue gas 214 increases the momentum of partially combustedgaseous product 216, while reducing the amount of recycled flue gas 214reduces the momentum.

Referring to FIG. 6, in yet another embodiment, secondary reactor 206may include at least one oxygen injector 223 providing the stream oftertiary oxygen 225 (see also FIG. 1) immediately adjacent to the areabounding a secondary reactor expellant 222 being transported fromsecondary reactor 206 to furnace 208 (not shown in FIG. 6). The streamof tertiary oxygen 225 illustrated in FIG. 6 enables increased controlover the properties of the reacting mixture exiting secondary reactor206, which can afford certain performance advantages. For example, itmay in certain circumstances be advantageous to operate the system witha stoichiometric ratio less than 1.0 at the exit of secondary reactor206 instead of adding tertiary oxygen to complete the combustionprocess. Operation in this manner should extend the reaction zone (orflame) farther into furnace 208, thereby lowering the peak temperatureand creating a more evenly distributed heat release pattern. Delaying ofthe completion of combustion also extends the life of transient, buthighly radiative species, in the flame. This enhancement of the radiantspecies of the flame will further assist in lowering the peak flametemperature and in improving the efficiency of heat transfer from theflame to the surroundings. The use of tertiary oxygen 225 is alsoadvantageous in that it can be introduced into secondary reactorexpellant 222 in such a way as to promote rapid mixing without theconstraint of overheating the secondary reactor walls. For example,tertiary oxygen 225 can be introduced through a plurality of swirl vanes702 as illustrated in FIG. 7, or through a plurality of convergingnozzles 802 as illustrated in FIG. 8. Those skilled in the art willappreciate that there are numerous other ways to introduce tertiaryoxygen within this disclosure.

Two additional embodiments which incorporate the use of tertiary oxygen225 are provided in FIGS. 9 and 10. FIG. 9 illustrates an embodimentwherein stream of quaternary oxygen 227 is premixed with recycled fluegas 214 prior to introduction into secondary reactor 206 (see also FIG.3). The embodiment in FIG. 10 does not include quaternary oxygen 227,but relies solely on oxygen in recycled flue gas 215 as the oxidizingagent. Since coal combustion system 202 is operated with some excessoxygen, recycled flue gas 214 typically includes oxygen. It will beappreciated that the two embodiments disclosed in FIGS. 9 and 10 resultin attenuation of the reactions within secondary reactor 206 comparedwith embodiments wherein undiluted secondary oxygen 224 is introducedadjacent to partially combusted gaseous product 216 exiting primaryreactor 204 (not shown in FIG. 10).

Systems employing this disclosure operate in a dynamic or changing mode.Moreover, a plurality of reactors (204 and/or 206) may operate inparallel. In such cases, maintaining optimal operation with low excessoxygen requires combustion instruments to measure properties associatedwith secondary reactor 206 in the system (i.e. local properties).Referring to FIGS. 11 through 14, secondary reactor 206 may include oneor more local combustion instruments 220 arranged and disposed toprovide information (including, but not limited to, feedback signals tothe fuel control system, the oxygen control system and/or the recycledflue gas control system) regarding conditions within secondary reactor206. For systems employing a plurality of secondary reactors 206, localcombustion instruments 220 generally are disposed on or within each ofsecondary reactors 206 in the system. Local combustion instrument 220may be selected from the group of instruments consisting of a flamescanner, a thermocouple, a non-intrusive instrument such as a tunablediode laser, an optical or acoustic sensor, and other instruments. Localcombustion instrument 220 may provide information including, but notlimited to, fluid temperature, fluid composition, and/or temperature ofportions of furnace 208. Local combustion instruments 220 may be locatedat any point within secondary reactor 206 or furnace 208 permittinglocal combustion instrument 220 to measure properties of secondaryreactor expellant 222 discharged from secondary reactor 206 to furnace208. Information from local combustion instrument 220 may subsequentlydeliver a control signal based upon the measurement by local combustioninstrument 220. The control signal may result, for example, inadjustment of the individual flow rate of secondary oxygen 224 ortertiary oxygen 225 to the radiant to secondary reactor 206. Asillustrated in the embodiment in FIG. 11, local combustion instrument220 may include a transmitter 223 and/or a receiver 233. As illustratedin the embodiment in FIG. 12, local combustion instrument 220 mayinclude a thermocouple 235.

In systems operating with a plurality of secondary reactors 206operating in parallel, an additional “global” combustion instrument 902is included to sample the mixed products of combustion, in particularthe concentrations of excess oxygen and carbon monoxide (CO), from allof secondary reactors 206. Such an embodiment is illustrated in FIG. 15.In this embodiment, the control system is configured to take both thelocal and global measurements as input. Signals from global measurementinstrument 902 are used by a controller 905 to determine whether or notmore or less total oxygen is required, while signals from localcombustion instruments 220 are used to determine the balance ofcombustion conditions among all parallel reactors. If, for example,controller 905 indicates that more oxygen is needed to improvecombustion efficiency (i.e. lower CO emission), then local combustioninstruments 220 are examined to determine which of secondary reactor(s)206 require the additional O₂. It will be appreciated by those ofordinary skill in the art that this mode of control permits balancing ofmultiple secondary reactors 206 operating in parallel, which will inturn facilitate efficient combustion with minimal excess oxygen.

Another mode of operation of the embodiment illustrated in FIG. 15includes the use of stream of quaternary oxygen 227 (see also FIG. 2)and/or recycled flue gas 219 in a region of furnace 208 downstream ofsecondary reactors 206. This would be desirable for operating with astoichiometric ratio below 1.0, as may be necessary after addition oftertiary oxygen 225. Such an operating mode should be advantageous, forexample, to reduce emissions of NO_(x). The use of the recycled flue gasin this area has a two-fold advantage. First, it may help to promotemixing of gases. Second, it may be used in a boiler with a convectivepass section 912 downstream of furnace 208 radiant section to adjustfurnace 208 exit gas temperature and flow rate to optimize the transferof the flue gas energy to produce steam. Quaternary oxygen 227 andrecycled flue gas 219 may be introduced as separate streams or premixedand introduced as a composite stream or streams. As illustrated in FIG.15, the embodiment of coal combustion system 202 includes heatexchangers in convective pass section 912. The heat exchangers mayinclude a secondary superheater 914, a reheat superheater 916, and aprimary superheater 918. In another embodiment, an economizer is alsoincluded. In yet other embodiments, additional heat exchangers may beincluded.

Another embodiment of the disclosure is illustrated in FIG. 16. In thisembodiment, primary reactor 204 delivers a partially oxidized gas streamto a plurality of secondary reactors 206 configured as described above.It will be appreciated by those of ordinary skill in the art that anadvantage of this embodiment is the reduction in solid fuel handling,metering and transport equipment needed compared to a system whereinmultiple primary reactors are employed. A further advantage issimplification of the balancing of secondary reactors 206. This isbecause a principal cause of secondary reactor 206 imbalance in a systememploying a plurality of primary reactors 204 is the relative imbalanceof fuel and oxygen flows to each of primary reactors 204. Thesimplification of secondary reactor 206 balancing operation should leadto improved system reliability and the ability to achieve completecombustion at even lower excess oxygen level than attainable in coalcombustion system 202 including a plurality of primary reactors 204.

While the disclosure has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1. A combustion system comprising: a primary reactor arranged anddisposed to receive a solid fuel and a first oxygen stream and deliver afirst substantially gaseous product and a substantially solid or moltenproduct; a secondary reactor in fluid communication with the primaryreactor; disposed to receive a second oxygen stream thereby convertingthe first substantially gaseous product from being oxygen deficient uponentering the secondary reactor to oxygen rich upon exiting the secondaryreactor; and a furnace in fluid communication with the secondaryreactor.
 2. The system of claim 1, wherein at least one of the oxygenstreams is substantially devoid of fluids selected from the groupconsisting of air, nitrogen, and combinations thereof.
 3. The system ofclaim 1, wherein at least one of the oxygen streams comprises at leastabout 30% by weight O₂.
 4. The system of claim 1, wherein the primaryreactor is configured to operate with a stoichiometric ratio of lessthan 1.0.
 5. The system of claim 1, wherein the secondary reactor isconfigured to operate with a stoichiometric ratio of greater than 1.0.6. The system of claim 1, wherein the combustion system is configured tomeasure one or more properties of the combustion gases that can becorrelated with the overall stoichiometric ratio of the system.
 7. Thesystem of claim 1, wherein the primary reactor is configured to operatewith a stoichiometric ratio of less than 0.95.
 8. The system of claim 1,wherein the secondary reactor is configured to operate with astoichiometric ratio of between 1.0 and 1.10.
 9. The system of claim 1,wherein the secondary reactor is configured to operate with astoichiometric ratio of between 1.0 and 1.05.
 10. The system of claim 1,wherein the partially combusted gaseous product is substantiallysurrounded by an oxidizing gas within at least a portion of thesecondary reactor.
 11. The system of claim 10, wherein the oxidizing gasincludes oxygen.
 12. The system of claim 10, wherein the oxidizing gasincludes recycled flue gas.
 13. The system of claim 10, wherein theoxidizing gas comprises both oxygen and recycled flue gas.
 14. Thesystem of claim 1, wherein the second oxygen stream is mixed with thepartially combusted gaseous product as it exits the secondary reactor.15. The system of claim 1, wherein the primary reactor is configured topermit a residual solid material in a molten state and substantiallyfree of residual carbon to be removed from it.
 16. The system of claim1, further comprising a recirculator arranged and disposed to permit aflue gas to be transported from a flue to the primary reactor.
 17. Thesystem of claim 1, wherein a local combustion instrument is arranged anddisposed to provide information on conditions within the secondaryreactor and/or of an expellant fluid from the secondary reactor.
 18. Thesystem of claim 1, further comprising at least one additional secondaryreactor in fluid communication with the primary reactor; wherein theadditional secondary reactor and the first secondary reactor arearranged and disposed to communicate at least one stream of an oxidizinggas to the first substantially gaseous product, wherein the additionalsecondary reactor and the first secondary reactor are in fluidcommunication with the furnace, wherein the primary reactor isconfigured to operate with a stoichiometric ratio of less than 1.0,wherein the secondary reactor is configured to operate with astoichiometric ratio of greater than 1.0, and wherein the combustionsystem is configured to measure at least a property of the combustiongases that can be correlated with the overall stoichiometric ratio ofthe system.
 19. The system of claim 1, wherein the furnace is arrangedand disposed to receive a fluid selected from the group of fluidsconsisting of oxygen, recycled flue gas, and combinations thereof,wherein the furnace is arranged and disposed to receive the fluid in aportion of the furnace substantially removed from the secondary reactorsthereby permitting an overall stoichiometric ratio of the system greaterthan 1.0.
 20. A combustion system as in claim 1, further comprising: alocal combustion instrument arranged and disposed to provide informationselected from the group consisting of conditions within the secondaryreactor, conditions of an expellant fluid from the secondary reactor,and combinations thereof; a global combustion instrument arranged anddisposed to provide information on conditions within the flue gas at apoint downstream from the furnace; and a controller arranged anddisposed to controllably provide oxygen streams in response to theinformation.
 21. A method of operating a combustion system comprising:providing a primary reactor arranged and disposed to receive a solidfuel and a first oxygen stream and deliver a first substantially gaseousproduct and a substantially solid or molten product; providing asecondary reactor in fluid communication with the primary reactor;disposed to receive a second oxygen stream converting the firstsubstantially gaseous product from being oxygen deficient upon enteringthe secondary reactor to oxygen rich upon exiting the secondary reactor;providing a furnace in fluid communication with the secondary reactor;and determining a stoichiometric ratio selected from the groupconsisting of the stoichiometric ratio of the primary reactor, thestoichiometric ratio of the secondary reactor, the stoichiometric ratioof the furnace, and combinations thereof.
 22. The method of claim 21,further comprising: providing a controlled amount of oxygen to theprimary reactor to maintain a stoichiometric ratio of less than about1.0; and providing a controlled amount of oxygen to the secondaryreactor to maintain a stoichiometric ratio of greater than about 1.0.23. The method of claim 22, further comprising operating with thestoichiometric ratio of the primary reactor below 0.95.
 24. The methodof claim 22, further comprising operating with the stoichiometric ratioof the secondary reactor between 1.0 and 1.10.
 25. The method of claim22, further comprising operating with the stoichiometric ratio of thesecondary reactor between 1.0 and 1.05.